We assessed a group of patients with a family history of Parkinson's disease (PD) in order to see if they differed clinically from sporadic cases and to study their genetic characteristics. Index cases were selected on the basis of clinically typical PD, and at least one affected relative. Fourteen families including 110 first degree and 40 second degree relatives were ascertained. A total of 31 individuals (17 females and 14 males) were found to be affected. This group was compared for selected clinical parameters to 31 age matched patients with sporadic PD. No statistical difference was found between the two groups. In familial cases, both the clinical parameters studied and the course of the disease varied within and between families, as observed in sporadic cases. The genetic transmission was compatible with an autosomal dominant model. The total segregation ratio of 0.25 suggested an incomplete penetrance, which increased with age, from 0 below the age of 30 to 0.43 over the age of 70. Age at onset was earlier in children than in their parents in the 8 multigeneration kindreds studied (mean difference 26 +/- 4.6 years, p = 0.01), whereas it was identical within a generation (mean difference 4.7 +/- 5.7 years, p = 0.1). Although we cannot exclude an ascertainment bias, our results are compatible with an anticipation phenomenon, which deserves further studies for confirmation.